Many people, particularly those who buy their pigs from petstores, don't understand why it's so important, so I want to elaborate a little on the previous link.

Quarantining a new pig that comes into a home where there are other pigs is essential in protecting your existing pig's health. The only time you don't have to do this if your pig comes from a reputable rescue where the animal has already been quarantined for three weeks.

Animals that come from petstores, breeders, or private individuals should all be quarantined regardless of what is said. The only exception would be adopting an animal where you know the owner and the surroundings, and you've had the opportunity of observing the animal for some weeks in a healthy environment. If in doubt, don't take a risk.

What it means:

Keeping the new animal for three weeks (some people do two) behind closed doors
Changing clothes/wearing a covering garment and wearing surgical gloves/handwashing between handling pigs. Remember that long hair can carry parasites and other diseases, so tie it back.
Bed on disposable bedding in the quarantine room
Sterilizing food bowls, water bottles, etc., separately from your healthy animals bowls and water bottles
Keeping children and other people out of the quarantine room if they are unlikely to follow strict quarantine procedures

What it doesn't mean:

Placing cages side by side or in the same room
Allowing the animals to 'just touch noses'
Introducing the animals 'just for a moment'
Putting the new animal's food or water dishes into the cage of the other animal, unless you've properly sanitized them
Thinking 'oh he/she looks just fine, and the petstore said the health was guaranteed'

Why:

Why expose your healthy animal to a host of other diseases which could be carried in by the new guinea pig? If you're lucky, you're just preventing mites/lice from spreading to your other animals, but you need to be aware of the other dangers:

URIs are very common - particularly in petstore pigs. A not insignificant number show signs of serious illness after purchase and some die. Without immediate treatment, most will die if signs of illness are ignored.

Ringworm (contagious to humans and other animals)

Fungal infections

Other airborne diseases, such as Bortella (will kill quickly)

During this quarantine period you can:

Have a thorough vet check

Treat with extra vitamin C, either by syringe or sprinkled on food (do not put in water)
Weigh regularly - daily is best - and bring up weight if necessary
Get the new animal used to fresh veggies - start slowly
Observe carefully and watch for signs of a URI or other illness
Check for blood on bedding, or other signs of stones or sludge such as squeaking while defecating
Get to know your new animal's particular personality

No, it's not convenient - in fact it's a royal pain. But it only goes on for 3 weeks, and it will help protect your existing animals from diseases or parasites brought in by the newcomer.

Furthermore, quarantining one animal can save you vet bills for multiple animals down the road.

Social issues also apply, as I learned with the addition of The Little Rotten One, who had a brief and obviously unpleasant (I found his food dish to be a used dirty ashtray, which is beyond even my original ignoramous state, when it comes to pigs!) pit stop with an owner who apparently thought he was getting a dog, but was originally the result of a mass breeding "pet" store chain well known for not caring about animals, he had to learn how to be handled with TLC.
He expected his humans to mistreat him and he was used to the constant high anxiety state that is caused by 20 to a fish tank mentality.
These little guys need a chance to emotionally decompress, if you will, to, LOL, "take a load off."
That little fart was a completely different animal three weeks after we got him.
They are much more ready to properly socialize with other piggies after a cooling off period.

I was told on a different forum that you would have to quarantine a adoptive pig that was from a shelter. I was thinking, well what if the pigs were seen first by a vet, treated and then put together? Would I still need to quarantine them?
And then I also thought of babies that you would get off Craigslist who are like 8 weeks old. Would you still quarantine them?

I was thinking, well what if the pigs were seen first by a vet, treated and then put together? Would I still need to quarantine them?

I think it's still better to quarantine them. While it's a great idea to take them to a vet, vets still can't catch everything--quarantine makes things doubly safer. An exception might be if the adopted pig is from a private rescue and has already been quarantined and treated for medical problems after being observed for three weeks in a foster home setting. But if they have come from a breeder, pet store, or animal shelter, you don't know if they have caught anything, or if it has been treated--so quarantine is a good idea.

I know this is good advice, my newest pigge has just died from pneumonia, and although he waas in a seperate cage from my girls, now i'm worried they might get it too.
I thught my pet shop looked after the animals! He was a rescue piggie
xxx

I just thought I would throw my two cents in on the quarantine topic. I just purchased a 5 week old pig from a reputable breeder. Thankfully I put her in quarantine, because 2 weeks in to it I found small white nits on her hair and lice on her belly. Since she is isolated I only have 1 pig to treat and not 9!

I have a 7 week old baby texel that has lice. Can anyone tell me the safest treatment for them? My vet typically uses ivermectin, but as this my first young animal I would like to make sure its the right choice.

Wow, yeah, it's pretty hard to miss lice. Shame on that breeder. (Well, shame for contributing to the overpopulation problem and risking the lives of her pregnant pigs, too. But, extra shame for the lice on a baby on top of that! Geez!)

Thats for the advice. I do understand that everyone on this forum is down on purebreds, but please do not be so judgemental. for the person who told me to adopt from a shelter- I have 4 pigs adopted from a local shelter, 3 rescued from a show, one rescued from a bad home and I have a revolving door set up with a local place that retires their older pigs to my house.

When I went to visit my parents during Christmas, I let a breeder babysit my guinea pigs. She put them in a tiny cage with her own pigs. They came back underweight, with parasites, and with wounds everywhere after being chased around for two weeks by the more dominant piggies. When I called to ask how they were doing she told me they were "doing just fine". She never mentioned she had put them with other piggies. I'm just happy they didn't end up pregnant.